about car chassis

Me and my team is going to design a solar powered car. And for that I am involved in making the chassis part that is to be made with steel in tubular frame due to more stability and low cost, reliability etc.

Can anyone suggest me something about it?
How i can start this and which type of steel i can use that is most stiff and reliable as the weight is going to be lifted by it around 400kg. Also the joining method as we want to make a much stable Tubular chassis.

Please make it as soon as possible.